You can go back to the early worlds and play them again at any time, and with all of the upgrades you have now you can run through them pretty easily. This is a good way to earn coins. The only thing to remember is not to let the enemies catch you off guard, because while they might be weak compared to how they used to be (because of your upgraded equipment), their speed can still catch you off guard and cause you to lose a level.
The most important reason to go back to the old levels is to go hunting for relics. Relics will give you a permanent boost, they are totally free, and they will be added to your collection forever. They will drop randomly on any stage (specific relics for specific levels), so you have to pick them up when they do in order to activate them. They will provide hero bonuses as well as bonuses for the fairy and the tower.
Hunting for relics is the way to get five stars on levels. The more relics you earn, the more stars you get – with one relic being worth half a star apiece (meaning that, obviously, there are ten relics on every level). Use the time lapse cheat, as mentioned in part 1 of the Tap Quest tips and tricks guide, to give yourself more lives whenever you need to so that you can go relic hunting for a long long time.
Know the best times to use your power ups that you pick up during levels. The critical hit is best done when you are fighting a boss. The lightning and fire are excellent from levels 5-10 because they take down huge crowds, very quickly. The leaf shield is great to buy time, but it’s as good as useless unless you put it directly over your gate.
Be sure to upgrade your power ups too. While this might seem obvious to some, to others it seems like a useless pursuit because of the seemingly small boosts that they get. However, the upgrades are cheap, so once you hit world 3 or 4 you should be able to mass upgrade some of them, making them far more powerful and useful. They will last longer and their attack will be stronger and – in the case of the shadow warrior – after enough upgrades you can make a second warrior appear when you use it.
Even if you are having bad luck with the silver chest, keep opening more and more of them. You might get five crappy items in a row, and then one high-tier item, and the one high-tier item will make it all worth it. If you have two or more gold items then this will actually be much riskier, though, as compared to gold items, silver items have fewer and smaller bonuses.